The present invention relates to a low-voltage circuit breaker, such as a molded case circuit breaker or an earth leakage breaker, and in particular, to an improvement for preventing metal melt generated during current interruption from spreading to an opening and closing mechanism.
In a circuit breaker, power-supply-side and load-side terminals are generally placed at respective longitudinal ends of a box-shaped insulated container, and the insulated container accommodates a conductor forming an electric path joining the terminals together; movable and fixed contact shoes that open and close the electric path; an arc-extinguishing chamber surrounding the movable contact shoe; an opening and closing mechanism for opening and closing the movable contact shoe via a holder made of an insulating material; and an overcurrent trip device for tripping the opening and closing mechanism.
FIG. 10 is a vertical sectional view showing an example of a conventional circuit breaker, i.e. three-pole circuit breaker, in its ON state, and FIGS. 11(A) and 11(B) are enlarged front views taken along line 11A,B--11A,B in FIG. 10, wherein FIG. 11(A) is an ON state and FIG. 11(B) is an OFF state.
In FIG. 10, a power-supply-side terminal 3 and a load-side terminal 4 for each pole are placed at the longitudinal (lateral direction in FIG. 10) ends of a box-shaped insulated container, which is formed of a case 1 with its top surface open and a cover 2 with its bottom surface open. An electric path between the terminals includes a fixed contact shoe 5 integrated with the power-supply-side terminal 3, a movable contact shoe 6 with one end contacting the fixed contact shoe 5, a lead wire 7 connected to the other end of the movable contact shoe 6, a heater conductor 9 in an overcurrent trip device 8 connected to the lead wire 7, and a relay conductor 10 connected to the heater conductor 9 and integrated with the load-side terminal 4. Fixed contact 5a and movable contact 6a are attached to the contact ends of the fixed and movable contact shoes 5, 6, respectively.
The movable contact shoe 6 is held at the right end in FIG. 10 in a cantilever manner by means of a holder 11 made of an insulating material, and the holder 11 is rotatably supported on the case 1 via an opening and closing shaft 12 that integrally connects the holders 11 for the respective poles. An opening and closing mechanism 13 is connected to the holder 11 for the central pole, and a switching handle 14 protruding from the cover 2 is moved to the right or left direction of FIG. 10 for an opening and closing operation to rotationally move the movable contact shoe 6 via the holder 11 so as to open and close in the direction shown by the arrow in the figure. In addition, when an overcurrent is generated, the overcurrent trip device 8 is activated to unlatch the opening and closing mechanism 13 via a trip cross bar 15 in order to automatically open the movable contact shoe 6. When the movable contact shoe 6 is opened, an arc occurs between the movable and fixed contacts 6a and 5a. To extinguish this arc, an arc-extinguishing chamber 16 is installed so as to surround the movable shoe 6.
The arc-extinguishing chamber 16 is composed of a plurality of magnetic plates or grids 17, each having a V-shaped notch and supported on a support plate 18 made of an insulating material. The arc-extinguishing chamber 16 withdraws the arc into the grids 17, where it is separated and cooled for extinction. The overcurrent trip device 8 in FIG. 10 is electromagnetically or thermally driven in a well-known manner. In an overcurrent condition, a bimetal in the overcurrent trip device 8 bends to rotationally move the trip cross bar 15, while in a high current condition including a short-circuit current, the device 8 instantaneously attracts a movable iron core to rotationally move the trip cross bar 15.
In the above circuit breaker, during the current interruption, arc heat melts the metal parts exposed to the arc, for example, the contacts 5a, 6a, the movable contact shoe 6 and the grids 17, and the metal melt splashes therearound due to a gas pressure generated by the arc heat.
Thus, a bulkhead or separating wall 19 formed of an insulated plate is conventionally inserted between the arc-extinguishing chamber 16 and the opening and closing mechanism 13 to prevent the metal melt from entering the opening and closing mechanism 13. When the metal melt adheres to the opening and closing mechanism 13, its movable part may be hindered from movement and even the opening and closing operations may be disabled.
As shown in FIGS. 11(A) and 11(B), the bulkhead or separating wall 19 is shaped like a gate having a slit 20 in the moving direction of the movable contact shoe 6, and is installed to separate an electric-path space for each of the poles partitioned and formed in the case 1 and cover 2 by means of inter-phase bulkheads. FIG. 11(A) shows an OFF state and FIG. 11(B) shows an ON state. As seen in FIG. 11(A), during the OFF state, the slit 20 forms a gap shown by shading in the lower part of the movable contact shoe 6, whereas during the ON state as shown in FIG. 11(B), the slit 20 forms a gap in the upper part of the movable contact shoe 6. Thus, conventionally, the bulkhead 19 can not sufficiently preclude the metal melt from entering the opening and closing mechanism 13, so that the opening and closing mechanism 13 has often been hindered from normal operations.
An object of this invention is to provide a circuit breaker that prevents metal melt generated by arc heat from entering the opening and closing mechanism.